Bridge Approach Section

I have questions about the use of 29” bridge rail & 28“Bridge Approach Section (BAS) taper to 31” over 50’

Has any thrie-beam BAS been tested at 28”?Should it work properly?

Keywords

Approach Guardrail Transitions

Other Keywords

none

Date

September 17, 2015

Response

Response

We have been asked previously
about reduced height transitions. We would expect that a slightly lower
transition height would allow for redirection of the vehicle. However, all
thrie beam transition testing that I have seen was conducted at 31" or at
the metric height of 31 5/8". In addition, lower transition heights
typically would expose more of the concrete barrier at the downstream end of
the transition and create a potential snag hazard. This would not be an issue
for you as you are using lower bridge heights as well.

With respect to transitions, the
issue has not been adequately addressed with respect to the current MASH
vehicles and impact conditions.

We cannot say with certainty
that reducing the rail height of the MGS transition will not affect its
performance. Reducing the height of the transition 2"-3" will
increase the impact load height on the post and the relation of the vehicle
front to the barrier. This in turn could increase the moment on the posts and
affect the lateral deflection and stiffness of the system. In addition, there
would be concerns for vehicle stability as well as an increased potential for
wood posts in the transition to fracture. Thus, we would generally recommend a
height of 31" for the MGS transition as well.

We have noted that for TL-2
applications, there is increased potential that a reduction in the rail height
for the transition of two inches would likely be acceptable.

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